Underground water is utilized as raw water for tap water and also utilized in industries which require a large quantity of water such as food, soft drink, beverage, dyestuff industries and also in public baths. In these industries, iron and manganese contents contained in underground water have raised a problem. While iron and manganese are substances which are necessary for a human body, contents of these substances exceeding a certain amount give a metal taste to water and turn water to red or black water, thereby making the water unsuitable for drinking and causing various difficulties in these industries. Further, in a foundation work in building industry, it is indispensable to remove underground water from a foundation site before the foundation work starts. If a large quantity of iron and manganese is contained in the underground water, the iron and manganese must be removed from the underground water before the underground water is discharged to the sewerage because it is legally prohibited to discharge such underground water the sewerage without any treatment.
In a currently prevailing conventional water treatment device for removing iron or manganese, an oxidizing agent such, for example, as sodium hypochlorite or a flocculant such, for example, as poly-aluminum chloride (PAC) is added to raw water to oxidize iron or manganese which is dissolved in the raw water and thereby convert it to iron oxide or manganese oxide which is insoluble in water and the iron oxide or manganese oxide is filtered out by filtering the raw water through filtering sand.
In the conventional water treatment device requiring addition of an oxidizing agent or a flocculant, however, a relatively large amount of oxidizing agent or flocculant is consumed in the device and, therefore, the cost of such oxidizing agent or flocculant is tremendous.
Further, since hypochlorous acid which is used as the oxidizing agent remains in water after the treatment for oxidizing iron and manganese, trihalomethane which is a carcinogen is generated and, for removing trihalomethane, the water must further be filtered through an activated carbon layer which adds to the cost of the water treatment. If provision of such activated carbon layer is omitted for economic reason, water after filtering must be constantly analyzed for preventing generation of trihalomethane caused by addition of an excessive amount of oxidizing agent and, if necessary, the amount of addition of the oxidizing agent must be adjusted. This method requires a high cost of maintenance in addition to the cost of purchasing the oxidizing agent.
Further, the conventional water treatment device generally is a complex and large-scale system including an aeration tank, a flocculation tank, a precipitation tank, a sand filter tower, an iron and manganese removing tower and a chemical agent tank and this system requires a large space for installation. It is impossible to install such a large device in a site of a limited space such in a town.
Furthermore, filtering sand which is used in this water treatment device requiring addition of an oxidizing agent is blocked by accumulated impurities and therefore must be replaced from time to time. The used sand to be abandoned must be treated as industrial waste because it contains a chemical agent and a place where it can be abandoned is extremely restricted by laws and regulations.
For eliminating the disadvantages of the prior art water treatment apparatus using a chemical and providing a water treatment apparatus which can oxidize iron, manganese and other elements dissolved in water such as underground water to make them insoluble in water with a simple and compact device and without using a chemical such as an oxidizing agent or a coagulant, Japanese Patent Application Laid-open Publication No. 2002-126768 discloses a water treatment apparatus in which raw water is jetted out by jet nozzles in which air is introduced through an air inlet or air inlet tube. A jet water stream including multitudes of air bubbles is blown out of the raw water jetting outlet and is struck against the water surface above the filter layer disposed below the raw water jetting outlet thereby causing vehement aeration both in the water above the filter layer and on the surface of the filter layer. By virtue of this aeration, soluble substances such as iron and manganese contained in the water are oxidized and thereby are turned to insoluble substances are caught on the surfaces of particles of the filter material such as filtering sand which constitutes the filter layer. Accordingly, dissolved substances such as iron and manganese in raw water can be turned to insoluble substances and filtered out with a simple and compact apparatus.
In the prior art water treatment apparatuses including one disclosed in the above described publication, iron which is made insoluble and deposited in the form of a film on the filter material tends to be removed excessively in the lower portion of the filter layer and removed insufficiently in the upper portion of the filter layer when the filter layer is cleaned with cleaning water. As to removal of iron and manganese, iron is turned to the state of film by oxidation in the upper portion of the filter layer whereas manganese is caught on the surface of the particle of the filter layer by the action of microorganism in the lower portion of the filter layer as will be described later. If, in a case where concentration of iron in raw water is significantly larger than concentration of manganese, the filter layer as a whole is cleaned with a large quantity of cleaning water for removing iron caught in the upper portion of the filter layer, microorganism living in the lower portion of the filter layer for catching manganese will be washed away and, as a result, catching of manganese in a subsequent water treatment operation will be obstructed.
On the other hand, it will be advantageous for a water treatment apparatus to increase filtering speed per unit time because, by doing so, the size and space required for the apparatus will be reduced. Particularly, space for establishing a water treatment apparatus is limited because a location at which such apparatus can be established is limited. In a case where a large scale water treatment apparatus cannot be established, necessary amount of raw water must be filtered with a small apparatus established in a narrow space and, in this case, it is necessary to increase the filtering speed of the apparatus.
An important factor necessary for increasing the filtering speed of the water treatment apparatus is efficiency of cleaning of the filter layer of the water treatment apparatus.
In a water treatment apparatus, as a filter operation is continued, the filter layer, particularly its surface portion, is covered with flocks of oxides such as iron oxide and other substances as time elapses and the filter function of the filter layer is reduced. In this case, the filter operation must be suspended temporarily and cleaning of the filter layer must be conducted. Accordingly, unless cleaning of the filter layer is conducted efficiently, increase in the filtering speed cannot be achieved.
In the water treatment apparatus described in the above described publication, the raw water supply tube is reciprocated in a plane parallel to the surface of the filter layer and the blocked filter layer is released from the blocked state by a jet water stream containing multitudes of air bubbles which is jetted out of the raw water supply tube to recover the filter function. Further, in this water treatment apparatus, a filter layer support made of a plate screen is provided in the bottom of the filter layer made of a single layer containing filtering sand and a reverse stream cleaning water supply tube for reverse stream cleaning the filter layer with reverse stream cleaning water is disposed below the filter layer support. When the upper portion of the filter layer, particularly its upper surface, has been covered with flocks of oxides and other substances, supply of raw water to the filtering tank is temporarily suspended and reverse stream cleaning water is caused to flow upwardly from the filter layer support to the entire filter layer to remove the substances covering the upper portion of the filter layer and wash away iron and manganese caught by the filter layer from an overflow outlet to the outside.
The filtering speed of this water treatment apparatus is 60 m/day to 130 m/day. In a case where it is necessary to increase the filtering speed of filter operation, cleaning of the filter layer must also be made more frequently and burden to treatment imposed by microorganism in the lower portion of the filter layer also increases. Therefore, efficiency of cleaning of the filter layer by this water treatment apparatus is limited to the above described filtering speed and a higher filtering speed cannot be achieved.
The prior art water treatment apparatus including one described in the above described publication must consume about 10% to 15% of received water (filtered water) for the above described cleaning operation with the result that efficiency of water treatment is significantly reduced.
The above described prior art water treatment apparatus has a mechanism for reciprocating the raw water supply tube in a plane parallel to the surface of the filter layer for cleaning the surface of the filter layer. Since this mechanism must reciprocate the raw water supply tube with an electric motor, a large power is required for cleaning the surface of the filter layer and a complex mechanism for reciprocation of the raw water supply tube. Further, this water treatment apparatus requires much labor and cost for maintenance of the apparatus including compensation for wear of rails for reciprocating the raw water supply tube and wear of a hose for supplying raw water to the raw water supply tube. Furthermore, for moving the raw water supply tube in which the cleaning nozzles are disposed in a line, the filtering tank must have a rectangular configuration with resulting increase in the size of the apparatus. Thus, it is difficult to make a compact design of the apparatus even in case the apparatus must be established in a narrow space.
Further, in the prior art water treatment apparatus in which cleaning of the filter layer is made by reciprocating the cleaning nozzles, the surface of the filter layer other than areas where the cleaning nozzles pass over remains uncleaned and hence it takes much time to complete cleaning of the entire surface of the filter layer resulting in difficulty in achieving a high speed cleaning of the filter layer.
It is, therefore, an object of the invention to provide a novel method for cleaning a water treatment apparatus capable of filtering water at a higher speed than the prior art water treatment apparatus and a water treatment apparatus which enables application of such method.
Since in the prior art water treatment apparatus, cleaning is made by reciprocating the raw water supply tube, cleaning on the surface of the filter layer other than an area where the raw water supply tube passes is not made and therefore considerable time is required before necessary cleaning is completed which poses a limit to a high speed treatment. It is therefore another object of the invention to improve this point.
It is another object of the invention to provide a water treatment apparatus which has an improved filtering efficiency by reducing the ratio of amount of cleaning water to the amount of filtered water and which does not require a large power and can be made in a simple, compact design requiring only a simple maintenance and a method for cleaning this water treatment apparatus.